Field of Invention
This invention relates generally to measuring the concentration of a dissolved gas present in a stream, and more particularly to a membrane-type probe for measuring the concentration of free or total chlorine, the probe being immersible in a process tank in a water or wastewater treatment system to provide a measurement that is unaffected by the pH of the water.
Chlorination is widely used to purify water supplies. In practice, chlorine is introduced at a selected point in the water supply system, and flow then takes place into a tank or through a region of flow which is sufficient for the chlorine to act effectively on the contaminants present in the water to produce a disinfecting action. In order to determine whether the amount of chlorine present is adequate to effect disinfection, measurements are made beyond the chlorine input point. The measurement output signal may also serve to regulate the feed of chlorine into the system to insure that the amount is adequate but not excessive.
The amount of chlorine added to the water is referred to as the "dosage," and is usually expressed as parts per million (ppm). The amount of chlorine used up or consumed by bacteria, algae, organic compounds and some inorganic substances, such as iron or manganese, is designated as the "demand." Since many of the reactions with chlorine are not instantaneous, but require time to reach completion, chlorine demand is time-dependent.
The amount of chlorine remaining in the water at the time of measurement is referred to as the "residual." Residual is therefore determined by the demand subtracted from the dosage. Inasmuch as chlorine demand is time-dependent, this dependency is likewise true of chlorine residual.
When chlorine dissolves in water, a mixture of hypochlorous and hydrochloric acids is formed. Actually, the hypochlorous acid dissociates into hydrogen and hypochlorite ions. In either the hypochlorous acid or hypochlorite ion form, chlorine is called "free chlorine residual." Free chlorine residual has a highly effective killing power toward bacteria.
Should the chlorinated water contain ammonia or certain amino (nitrogen-based) compounds, as is the case with sewage, then additional compounds, called chloramines, are formed. Chloramines occur almost instantaneously, and though several reactions are possible between hypochlorous acid and ammonia, chloramines collectively are referred to as "combined chlorine residual." This combined chlorine residual has a much lower bactericidal effect than free chlorine residual. The term "total chlorine" as used herein is the sum of free and combined chlorine.
The analysis of process water or wastewater in a treatment system for chlorine in its various forms (free, combined and total) has long presented problems. The typical continuous analyzer for this purpose requires that a sample be withdrawn from the process by a pump and delivered to the analyzer, at which point various chemical reagents are added to carry out the appropriate measurement.
The presence of suspended solids in wastewater usually dictates a filter system to exclude these solids from the sample to be tested, and this in turn given rise to maintenance problems. Maintenance is also called for in connection with the supply of various chemical additives to the analyzer, this operation entailing low capacity pumps and metering hardware. These factors, taken together with the cost of the chemicals that must be supplied to the analyzer on a 24-hour per day, year round basis, result in an analyzer that is relatively difficult and expensive to install, and one that is costly to operate and maintain.
U.S. Pat. No. 3,413,199 discloses a galvanic cell usable for free chlorine analysis, while U.S. Pat. Nos. 3,948,746 and 4,176,032 disclose dissolved oxygen and chlorine dioxide probes of the membrane type. A normal requirement of membrane-type probes, such as those disclosed in the above-identified patents, is that the sample being analyzed flow past the outer surface of the membrane at a relatively high velocity to maintain analytical sensitivity. For most commercially available membrane-type probes, the required minimum flow velocity is in the order of 0.5 to 1.0 foot per second.
The reason for this rapid flow requirement is that with a sample solution that is relatively static or still, the region immediately outside the membrane becomes depleted of the species being measured by reason of the transfer of this species through the membrane to the interior of the electrochemical cell incorporated in the probe. Unless the depleted region is replenished rapidly from the bulk solution, the analytical output or sensitivity of the instrument is markedly decreased.
In those situations in which the probe is immersed in a rapidly flowing sample stream, this problem does not arise. But in most cases, except for vigorously agitated aeration tanks in sewage treatment facilities, a high velocity stream is normally not the usual environment. Thus in a typical chlorine contact tank where it is necessary to measure chlorine concentration, one encounters very slow moving, unstirred streams.
It is important not to insert a membrane-type probe into a pipeline discharging from a pump, for membrane type probes are severely affected by variations in pressure applied to membranes. While pressure-compensated probes are available, these must be fully inserted in a pipe. As a consequence, the inserted probe introduces a major obstacle in the flow path which acts to collect debris, this leading ultimately to clogging and disabling of the probe.
Another frequently used approach toward imparting the necessary velocity to the sample stream is to install a mechanical stirring device in close proxmity to the membrane of the probe. But this falls short of a satisfactory solution to the problem, for such stirrers in a contaminated stream usually collect on their stirring surfaces suspended process solids, paper, string and other foreign substances. As a result, the stirring action is inhibited. Also, the accumulated material on the stirring blades may make contact with and rupture or otherwise impair the adjacent delicate membrane of the probe.